Diode-less array for one-time programmable memory

ABSTRACT

A one-time programmable memory array includes a first row conductor extending in a first row direction and disposed at a first elevation, a second row conductor extending in a second row direction and disposed at a second elevation and a column conductor extending in a column direction and disposed adjacent to the first row conductor and adjacent to the second row conductor. The array also includes a dielectric layer covering at least a portion of the column conductor, a fuse link coupled between the dielectric layer on the column conductor and the second row conductor.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of patent application Ser. No.11/297,529, filed on Dec. 18, 2005, which is now allowed. The entiretyof the above-mentioned patent application is hereby incorporated byreference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

The present invention relates to a diode-less array for a one-timeprogrammable memory and a method for manufacturing a diode-less arrayfor one-time programmable memory, and more particularly, to a diode-lessarray for a one-time programmable memory having a dielectric layer and amethod for manufacturing a diode-less array for a one-time programmablememory having a dielectric layer.

Erasable programmable read only memory (EPROM) is known in the art. AnEPROM is programmed electronically, usually by means of a programmingdevice for storing and downloading information. An EPROM can be erasedand reprogrammed. The EPROM typically includes a quartz glass window inthe package for erasing the contents by the application of ultraviolet(UV) light. When the quartz glass window is exposed to the UV light fora period of time, the EPROM is erased and can then be reprogrammed.

One-time programmable (OTP) memory is also known in the art. There areseveral types of OTP memory including fuse, anti-fuse, PROM and maskread only memory (mask ROM). Generally, the content of an OTP is created(programmed) by the customer rather than by the manufacturer. A PROM isa kind of storage device like an EPROM but with no quartz glass windowin the package for erasing the contents which reduces the packaging costbut means the device cannot be erased with UV and so can only be writtenonce. A PROM usually comes with all bits reading as logic “1” andblowing a fuse during programming causes each respective bit to read asa logic “0.”

Fuse OTP cells include a plurality of “fuses” that are selectivelyexposed to programming currents in order to burn-out selected fuses toachieve desired programming. Anti-fuse OTP cells use breakdown of metalinsulator or diode structures to create two differing resistance statesto achieve desired programming. An antifuse functions in an oppositemanner as the fuse which allows conduction up to a certain level. Anantifuse allows conduction up to a certain level and when that level isexceeded, the antifuse closes the conduction path thereby allowing lowresistance current flow through the antifuse.

Mask ROM is a type of OTP that is programmed during fabrication,therefore there is no programming circuitry necessary for a mask ROM. Asthe name implies, a mask ROM is created during semiconductor fabricationby selectively photomasking the fabricated device to achieve the desiredprogrammed state. However, programming a mask ROM becomes increasinglydifficult as memory cell size is further reduced. Additionally, the turnaround time (TAT) to manufacture an order may take several weeks becauseparticular masks have to be developed for each application.

It is desirable to provide a diode-less array for OTP memory. Further,it is desirable to provide an OTP memory array that has a dielectriclayer.

BRIEF SUMMARY OF THE INVENTION

Briefly stated, the present invention comprises a one-time programmablememory array that includes a first row conductor extending in a firstrow direction and disposed at a first elevation, a second row conductorextending in a second row direction and disposed at a second elevationand a column conductor extending in a column direction and disposedadjacent to the first row conductor and adjacent to the second rowconductor. The column direction being different from the first andsecond row directions. The array also includes a dielectric layercovering at least a portion of the column conductor, a fuse link coupledbetween the dielectric layer on the column conductor and the second rowconductor.

The present invention also comprises a one-time programmable memoryarray that includes a plurality of first row conductors extending in afirst row direction and disposed at a first elevation, a plurality ofsecond row conductors extending in a second row direction and disposedat a second elevation and a plurality of column conductors extending ina column direction and disposed between adjacent pairs of the pluralityof first row conductors and adjacent pairs of the plurality of secondrow conductors. The second elevation is different than the firstelevation. The array also includes a plurality of dielectric layers anda plurality of fuse links. Each of the plurality of dielectric layerscovers at least a portion of each one of the plurality of columnconductors. Each of the plurality of fuse links is coupled between oneof the plurality of column conductors and one adjacent row conductor ofone of the plurality of second row conductors.

In another aspect, the present invention comprises a method of forming aone-time programmable memory array having a dielectric layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there is shown in the drawings an embodiment which is presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown. In thedrawings:

FIG. 1 is a partial elevational cross-sectional view of a one-timeprogrammable (OTP) memory array having a dielectric layer in accordancewith the preferred embodiment of the present invention;

FIG. 2 is a perspective view of a intermetal dielectric (IMD) oxide baseand first set of row conductors for forming the OTP memory array of FIG.1;

FIG. 3 is a perspective view of the base of FIG. 2 with the addition ofa fuse layer and a second set of row conductors;

FIG. 4 is a is a perspective view of the base of FIG. 3 having the fuselayer patterned and with the addition of column conductors;

FIG. 5 is a is a perspective view of the base of FIG. 4 with adielectric layer added to the column conductors;

FIG. 6 is a perspective view of the OTP memory array of FIG. 1 withoutan insulator filling voids therein;

FIG. 7 is a top plan view of the OTP memory array of FIG. 1;

FIG. 8 is a perspective view of the OTP memory array of FIG. 6configured for programming one of a plurality of memory cells; and

FIG. 9 is a perspective view of the OTP memory array of FIG. 6configured for reading one of a plurality of memory cells.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used in the following description for convenienceonly and is not limiting. The words “right”, “left”, “lower”, and“upper” designate directions in the drawing to which reference is made.The words “inwardly” and “outwardly” refer direction toward and awayfrom, respectively, the geometric center of the object described anddesignated parts thereof. The terminology includes the words abovespecifically mentioned, derivatives thereof and words of similar import.Additionally, the word “a,” as used in the claims and in thecorresponding portions of the specification, means “at least one.”

As used herein, reference to conductivity will be limited to theembodiment described. However, those skilled in the art know that p-typeconductivity can be switched with n-type conductivity and the devicewould still be functionally correct (i.e., a first or a secondconductivity type). Therefore, where used herein, the reference to n orp can also mean that either n and p or p and n can be substitutedtherefor.

Furthermore, n⁺ and p⁺ refer to heavily doped n and p regions,respectively; n⁺⁺ and p⁺⁺ refer to very heavily doped n and p regions,respectively; n⁻ and p⁻ refer to lightly doped n and p regions,respectively; and n⁻⁻ and p⁻⁻ refer to very lightly doped n and pregions, respectively. However, such relative doping terms should not beconstrued as limiting.

Referring to the drawings in detail, wherein like numerals referenceindicate like elements throughout, there is shown in FIGS. 1-9 aone-time programmable (OTP) memory array 50 that includes a first rowconductor 52 extending in a first row direction D1 and disposed at afirst elevation E1, a second row conductor 54 extending in a second rowdirection D2 (shown as an arrow coming out of or going into the page inFIG. 1) and disposed at a second elevation E2 and a column conductor 56extending in a column direction D3 and disposed adjacent to the firstrow conductor 52 and adjacent to the second row conductor 54. The OTPmemory array 50 also includes a dielectric layer 60 covering at least aportion of the column conductor 56 and a fuse link 64 coupled betweenthe dielectric layer 60 on the column conductor 56 and the second rowconductor 52. The first row conductor 52 forms a word line and thesecond row conductor 54 forms a bit line of the OTP memory array 50.

The first row direction D1 and the second row direction D2 aredifferent. Preferably, the first row direction D1 and the second rowdirection D2 are orthogonal with respect to each other. Of course, thefirst and second row directions D1, D2 may be at other angles withrespect to each other

The fuse link 64 coupled between the dielectric layer 60 on the columnconductor 56 and the second row conductor 54 defines a memory cell 66which “stores” a binary state by programming. For example, when the fuselink 64 of a particular memory cell 66 is intact, then the particularmemory cell 66 may be a logic “0’ and, if the fuse link 64 of aparticular memory cell 66 is opened or “blown,” then the particularmemory cell 66 may be a logic “1,” or vice versa. Of course, once amemory cell 66 is “programmed,” by opening the fuse link 64 associatedwith that memory cell 66, that particular memory cell 66 cannot beunprogrammed because the fuse link 64 cannot be restored once opened orburned, thus demonstrating the one time programmable character of thememory array 50.

The dielectric layer 60 is preferably formed of a dielectric materiallike a nitride or an oxide. Optionally, the dielectric layer 60 may beformed by oxidizing the material of the fuse link 64. The fuse link 64is preferably formed of doped polysilicon, undoped polysilicon or a thinmetal. The first and second row conductors 52, 54 and the columnconductor 56 are formed of polysilicon or a metal such as copper,aluminum, germanium, tantalum, silver, gold, nickel, chromium, tin,tungsten, zinc, titanium, indium and the like or combinations thereof.

The OTP memory array 50 more particularly includes a plurality of firstrow conductors 52 extending in the first row direction D1 and disposedat the first elevation E1, a plurality of second row conductors 54extending in the second row direction D2 and disposed at the secondelevation E2, a plurality of third row conductors 54 extending in thesecond row direction D2 and disposed at a third elevation E3, aplurality of fourth row conductors 52 extending in the first rowdirection D1 and disposed at a fourth elevation E4 and a plurality ofcolumn conductors 56 extending in the column direction D3 and disposedbetween adjacent pairs of the plurality of first row conductors 52 andadjacent pairs of the plurality of second row conductors 54. The secondelevation E2 is above the first elevation E1, with respect to a base 51of the OTP memory array 50; the third elevation E3 is above the secondelevation E2, with respect to the base 51; and the fourth elevation E4is above the third elevation E3, with respect to the base 51. Thus, thefirst-fourth row conductors 52, 54 are in overlying relationship to oneanother.

The second row conductors 54 include bit lines BL1A, BL2A, BL3A andBLnA. The third row conductors 54 include bit lines BL1B, BL2B, BL3B andBLnB. The first row conductors 52 include word lines WL1A, WL2A, WL3Aand WLnA. The fourth row conductors 52 include word lines WL1A, WL2A,WL3A and WLnA. The column conductors 56 interconnect between adjacentword lines WL1A, WL2A, WL3A, WLnA and the fuse links 64 bridge toindividual bit lines BL1A, BL2A, BL3A, BLnA, BL1B, BL2B, BL3B, BLnB sothat each bit line BL1A, BL2A, BL3A, BLnA, BL1B, BL2B, BL3B, BLnB iselectrically coupled to each of the word lines WL1A, WL2A, WL3A, WLnA.Accordingly, the OTP memory array 50 also includes a plurality ofdielectric layers 60 and a plurality of fuse links 64 for making theinterconnections to the individual bit lines BL1A, BL2A, BL3A, BLnA,BL1B, BL2B, BL3B, BLnB. Each of the plurality of dielectric layers 60covers at least a portion of each one of the plurality of columnconductors 56. The word lines WL1A, WL2A, WL3A, WLnA, the bit linesBL1A, BL2A, BL3A, BLnA, BL1B, BL2B, BL3B, BLnB, the dielectricallycovered column conductors 56 and the fuse links 64 form athree-dimensional (3D) inter-layered matrix. Since the OTP memory array50 is a three-dimensional (3D) memory array code efficiency and memorydensity are both improved per unit area/volume as compared to a mask ROMand a conventional PROM that use diodes.

The dielectric layers 60 are used in lieu of a diode. The function of adiode in a OTP memory array 50 is to reduce parasitic current that flowsthrough other memory units 66 to the current sensor because diodes havelarge forward and small reverse current characteristics. The dielectriclayers 60 perform this function as well in lieu of a diode. If thedielectric layers 60 are formed sufficiently thin, the dielectric layers60 have a large tunneling current, especially direct tunneling current.The parasitic current becomes very small as the parasitic current flowsthrough other memory units 66 because it flows through many dielectriclayers 60 in its path. The tunneling current is expressed as follows:

$J_{DT} \cong {\frac{4\pi\;{qm}_{1}k_{B}T}{h^{3}}{\int_{0}^{E_{b}}{{{TC}(E)}{\ln\left\lbrack \frac{{\mathbb{e}}^{{{({E_{{Fn}\; 1} - E_{c\; 1} - E})}/k_{B}}T} + 1}{{\mathbb{e}}^{{{({E_{{Fn}\; 3} - E_{c\; 1} - E})}/k_{B}}T} + 1} \right\rbrack}{\mathbb{d}E}}}}$using a Wentzel-Kramers-Brillouin (WKB) calculation of the tunnelingcoefficient through a trapezoidal barrier (i.e., direct tunneling). TheWKB tunneling coefficient is given by,

${{TC}(E)} = {\exp\left( {{- \frac{4}{3}}\left( \frac{8\pi^{2}m_{2}}{h^{2}} \right)^{\frac{1}{2}}{\left( \frac{t_{ins}}{{qV}_{ins}} \right)\left\lbrack {\left( {E_{b\; 1} - E - {q\;\frac{V_{ins}}{t_{ins}}b}} \right)^{\frac{3}{2}} - \left( {E_{b\; 1} - E - {q\;\frac{V_{ins}}{t_{ins}}a}} \right)^{\frac{3}{2}}} \right\rbrack}} \right)}$where, E_(b1) is the barrier height on the incident side, m₂ is theeffective tunneling mass of electrons in the insulator and a, b areclassical turning points.

-   Fowler-Nordheim (FN) tunneling current is expressed as follows:

${Jdt} = {{A\left( \frac{V}{T_{ins}} \right)}^{2}{\exp\left\lbrack {- \frac{B}{\left( \frac{V}{T_{ins}} \right)}} \right\rbrack}}$where, A, B are constants, Tins is the dielectric thickness and V isbias across the dielectric 60.

The OTP memory array 50 may include any number of bit lines BL1A, BL2A,BL3A, BLnA, BL1B, BL2B, BL3B, BLnB and any number of word lines WL1A,WL2A, WL3A, WLnA. Moreover, there may be additional layers of bit linesBL1A, BL2A, BL3A, BLnA, BL1B, BL2B, BL3B, BLnB and word lines WL1A,WL2A, WL3A, WLnA that may be interconnected in different manners toachieve even higher density OTP memory arrays 50. OTP memory arrays 50may be several gigabytes (GB) or more. For example, a one GB OTP memoryarray 50 may be stacked by eight elevations so that the array 50includes 16 Million (M)*8 bit lines and 8M word lines.

Each memory cell 66 has two (binary) memory states: “programmed” and“un-programmed.” The un-programmed state is when the fuse link 64 isintact, and the programmed state is when the fuse link 64 is “blown”(i.e., an open circuit). Referring to FIG. 8, in order to program theOTP memory array 50, a programming voltage V_(PGM) is selectivelyapplied to a particular word line 52 and a particular bit line 54 isselectively connected to return (ground) or vice versa. The programmingvoltage V_(PGM) in conjunction with the resistance in the path to thereturn permits a current I_(PGM) to the return. The programming currentI_(PGM) is sufficient to cause the particular fuse link 64 between thedielectric layer 60 on the respective column conductor 56 and the bitline 54 to blow (i.e., open circuit). Once “programmed,” a particularbit line 54 cannot be “un-programmed.” For example, as shown in FIG. 8,a positive programming voltage V_(PGM) is applied to bit line BL2B and anegative programming voltage V_(PGM) is applied to word line WL1A.Current I_(PGM) flows through the path shown by the small directionalarrows including the word line WL1A, the column conductor 56, the fuselink 64 and bit line BL2B. The current is sufficient to blow the fuselink 64 connected between BL2B and the column conductor 56 therebyprogramming the particular memory cell 66 associated with bit line BL2B.

The programmed state may represent binary value “1,” and theun-programmed state may represent binary value “0.” Alternatively, theprogrammed state may represent binary value “0,” and the un-programmedstate may represent binary value “1.”

FIG. 9 shows one way that the memory array 50 may be read by applyingread voltage V_(read) to particular word lines WL1A, WL2A, WL3A, WLnAand bit line BL1A, BL2A, BL3A, BLnA, BL1B, BL2B, BL3B, BLnBcombinations. For example, as shown in FIG. 9, a positive read voltageV_(read) is applied to bit line BL2B and a negative read voltageV_(read) is applied to word line WL1A. Current I_(read) flows throughthe path shown by the small directional arrows including the word lineWL1A, the column conductor 56, the fuse link 64 and bit line BL2B. Acurrent detector (not shown) is used to sense the presence or absence ofread current I_(read). If the read current I_(read) is at an expectedlevel, then the fuse link 64 for the particular memory cell beingmeasured must be intact, and therefore, the memory cell 66 was notprogrammed (e.g., logic 0). But, if there is no measurable read currentI_(read), then the fuse link 64 for the particular memory cell beingmeasured must be blown, and therefore, the memory cell 66 was programmed(e.g., logic 1).

The word lines 52 may be coupled to a Y decoder (not shown) and the bitlines 54 may be coupled to an X decoder (not shown) for reading thestates of the various memory cells 66 within the OTP memory array 50 bymultiplexing or similar techniques.

FIGS. 2-6 demonstrate exemplary steps for fabricating the OTP memoryarray 50. FIG. 2 shows that the process begins with an intermetaldielectric (IMD) oxide base 51 on which a first set of row conductors 52are formed. A layer of conductive material can be grown or depositedonto the IMD oxide base 51 and then patterned and etched to form thefirst set of row conductors 52. A dielectric or insulating material isthen filled into the patterned spaces between the first set of rowconductors 52. Alternately, a layer of dielectric material 53 can bedeposited on the IMD oxide base 51, and the dielectric material 53 canbe patterned and etched for filling by conductive material to form thefirst set of row conductors 52 and then covered with the dielectricmaterial 53. FIG. 3 shows the IMD base 51 with the first set of rowconductors 52 of FIG. 2 with the addition of another IMD oxide layer 53,a fuse layer 63 and a second set of row conductors 54. The fuse layer 63may be a deposition of polysilicon forming a thin film. The fuse layer63 is patterned (striped) to create fuse links 64. The second row ofconducts 54 may be formed by depositing a relatively thick layer ofmetal or polysilicon and then patterning the material to createindividual bit lines BL1A, BL2A, BL3A, BLnA. Alternately, individual bitlines BL1A, BL2A, BL3A, BlnA can be created before the formation of fuselink. FIG. 4 shows the partially manufactured memory array of FIG. 3having the fuse layer 63 patterned (striped) to thereby form fuse links64 and with the addition of column conductors 56 and each of the columnconductors 56 must cut off each of the fuse links 64 into two parts.FIG. 5 shows the partially manufactured memory array of FIG. 4 with adielectric layer 60 added to the column hole. The dielectric layer 60 isattached to the sidewall of column hole. A conductive material such astungsten is filled into the column hole to form the column conductors56. FIG. 6 shows that another fuse layer 63 has been added above thesecond layer of row conductors 54 and that another layer of rowconductors 54 has been formed in order to create the OTP memory array 50of FIG. 1 without the insulator 57 filling voids therein. The fuse layer63 is patterned (striped) to create fuse links 64, and the second layerof row conductors 54 are patterned to create individual bit lines BL1B,BL2B, BL3B, BLnB. The insulating material 57 may then be added by refillor deposition and the like to yield the OTP memory array 50 of FIG. 1.Alternately, memory array 50 of FIG. 1 can be fabricated by anotherprocess flow. For example, the relative process of the dielectric layer60 and column conductor 56 showed in FIGS. 4-5 can be initially skippedto stack BL1A, BL2A, BlnA, BL1B, BL2B, BLnB, and then finally, createdielectric layer 60 and column conductor 56.

The process may be repeated a number of times to stack a plurality ofrow conductors 52, 54, fuse links 63, column conductors 56, bit linesBL1A, BL2A, BL3A, BLnA, BL1B, BL2B, BL3B, BLnB, and word lines WL1A,WL2A, WL3A, WLnA, thereby forming a larger OTP memory array 50 having aplurality of memory cells 66.

The various layers may be formed in any of a variety of ways known inthe art. For example, the layers may be grown or deposited. Depositionmay be by chemical vapor deposition (CVD), physical vapor deposition(PVD), evaporation, sputtering and the like. Patterns may be formed onthe surface of the semiconductor substrate by photolithography orphotomasking (“masking”) techniques. Layers may be etched back bymechanical etching or by chemical etching and/or chemical mechanicalpolishing and the like. Additionally, known methods of doping, heattreating, diffusing, etching, layering, trenching, polishing and thelike, may be utilized in the fabrication process of the OTP memory array50 without departing from the present invention.

From the foregoing, it can be seen that the present invention isdirected to a diode-less array for one-time programmable memory having adielectric layer and a method for manufacturing a diode-less array forone-time programmable memory having a dielectric layer. It will beappreciated by those skilled in the art that changes could be made tothe embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

What is claimed is:
 1. A memory array structure of a plurality of memoryunits, comprising: a word line; a column conductor connected the wordline; a dielectric material covering at least a portion of the columnconductor, wherein the dielectric material is continuous among theplurality of memory units; a plurality of bit lines disposed at aplurality of elevations; and a plurality fuse links, wherein each of theplurality of fuse links being coupled between the dielectric material onthe column conductor and one of the plurality bit lines.
 2. The memoryarray structure according to claim 1, wherein the dielectric material isone of nitride and silicon dioxide.
 3. The memory array structureaccording to claim 1, wherein the fuse link is formed of doped orundoped polysilicon.
 4. The A memory array structure according to claim1, wherein the first and second row conductors and the column conductorare formed of at least one of polysilicon, copper, aluminum, germanium,tantalum, silver, gold, nickel, chromium, tin, tungsten, zinc, titaniumand indium.
 5. The A memory array structure according to claim 1,further comprising: an insulator filling voids between the bit lines,the fuse links and the column conductor.
 6. The memory array structureaccording to claim 1, wherein a particular fuse link coupled between thedielectric material on the column conductor and a particular bit line isblown by applying a programming voltage on the word line and groundingthe particular bit line.
 7. The memory array structure according toclaim 1, wherein a particular fuse link coupled between the dielectricmaterial on the column conductor and a particular bit line is measuredby applying a negative read voltage on the word line and applying apositive read voltage on the particular bit line.
 8. A memory arraystructure of a plurality of memory units, comprising: a row conductorextending in a first row direction and disposed at a first elevation; abit line extending in a second row direction and disposed at a secondelevation; a column conductor extending in a column direction andcoupled to the row conductor and coupled to the bit line; a dielectricmaterial covering at least a portion of the column conductor, whereinthe dielectric material is continuous among the plurality of memoryunits; and a fuse link being coupled between the dielectric material onthe column conductor and the bit line; wherein the second elevationbeing different than the first elevation, and the column direction beingdifferent from the first and second row directions.
 9. The memory arraystructure according to claim 8, wherein the dielectric material is oneof nitride and silicon dioxide.
 10. The memory array structure accordingto claim 8, wherein the fuse link is formed of doped or undopedpolysilicon.
 11. The memory array structure according to claim 8,wherein the bit line, the row conductor and the column conductor areformed of at least one of polysilicon, copper, aluminum, germanium,tantalum, silver, gold, nickel, chromium, tin, tungsten, zinc, titaniumand indium.
 12. The memory array structure according to claim 8, whereinthe first row direction and the second row direction are orthogonal withrespect to each other.
 13. The memory array structure according to claim8, further comprising: an insulator filling voids between the bit line,the row conductor, the fuse link and the column conductor.
 14. Thememory array structure according to claim 8, wherein the row conductorforms a word line.
 15. The memory array structure according to claim 14,wherein the fuse link is blown by applying a programming voltage on theword line and grounding the bit line.
 16. The memory array structureaccording to claim 14, wherein the fuse link is measured by applying anegative read voltage on the word line and applying a positive readvoltage on the bit line.